GANDERIA - AN IMPORTANT PERI-GONDWANAN TERRANE IN THE NORTHERN APPALACHIANS

The tectonic evolution of the central core of the Northern Appalachians revolves around the formation, fragmentation, and accretion of Ganderia, a highly significant, but widely misunderstood paleogeographic element. Ganderia is primarily defined on the basis of its Cambrian-Lower Ordovician arenite-shale sequence that is disconformably overlain by volcanic rocks with an Arenig cold-water fauna and a high-latitude paleomagnetic imprint. Gander rocks underlie most parts of the Central Maine belt and the Fredericton Trough in New England, and the Central Mobile Belt in Newfoundland. Upper Neoproterozoic-Lower Cambrian arc plutonic- and volcanic rocks exposed in New Brunswick, Nova Scotia and Newfoundland (e.g. Upsalquitch, New River, Aspy, Valentine Lake, Hermitage Flexure) can be correlated with some degree of confidence with the unexposed basement beneath the arenite-shale sequence on the basis of inherited zircon populations, Nd-isotope data and geophysical data. In addition, an unconformity between Lower Cambrian rocks of the New River belt and a Gander-type arenite cover has been identified in southern New Brunswick. The arenite-shale sequence experienced ophiolite obduction (Penobscot) during the Late Cambrian/Early Ordovician in Newfoundland and probably also in New Brunswick and Maine. The Lower to Middle Ordovician Popelogan arc, built on the northern edge of Ganderia, arrived at the Laurentian margin in the Caradoc followed by the rest of Ganderia during the Ashgill and Llandovery. Accretion of Ganderia to Laurentia was rapidly followed by the accretion of Avalon s.s. (e.g. Mira, Caledonia, Boston), evidence of which is preserved in Silurian arc volcanic rocks on the southeastern margin of Ganderia in Nova Scotia, New Brunswick and Maine. The exact time of Avalon’s arrival is poorly constrained, but docking probably took place during the Late Silurian or Early Devonian. The docking of Avalon was followed by the arrival of Meguma, most likely a fragment of Armorica, during the Early Devonian. The Lower Devonian plutons, so prominent in the central and southern parts of Ganderia, probably formed when the two northwest-dipping slabs, related to closure of the Tetagouche-Exploits back-arc basin and the basin that separated Ganderia from Avalon, broke-off and were replaced by hot asthenosphere.